Abstract The focus of this proposal is to identify the role of posttranslational modifications of proteins and peptides in the development of an autoimmune T cell response. The hypothesis directing these studies is that inflammation induces localized posttranslational modifications (PTM) of proteins, and that these modifications induce epitope spreading by generating neo-epitopes which become focal points of autoimmune T cell stimulation that perpetuate the autoimmune disease. Recent clinical observations have implicated PTM proteins as potential autoantigens in RA and other autoimmune diseases. The posttranslational modification of arginine to citrulline has become a major focus of study in RA as the presence of antibodies to citrullinated proteins has become a diagnostic for this autoimmune disease. Recently, antibodies to homocitrulline, a PTM of lysine, have also been described. In our proposed studies, we will use humanized mouse models that express HLA-DR1 or DR4 alleles which are associated with susceptibility to RA, and study the role of PTM peptides in the development of autoimmune arthritis. Through the experiments in this proposal we will gain new insight into the evolution of an autoimmune T cell response mediated by these HLA-DR alleles and how the PTM of autoantigens is involved in the generation and function of autoimmune T cells at the site of inflammation. The studies in this proposal are focused on: a) the role of posttranslational modifications of autoantigenic peptides in generating neo-epitopes that stimulate T cells in autoimmune arthritis, b) the identification, specificity, and functional phenotype of these PTM neo-epitope-specific CD4+ T cells that are found within the inflamed arthritic joints, and c) the binding and presentation of the PTM peptides by RA susceptibility alleles DR1 and DR4 and the role of the shared eptiope in the presentation of these peptides to T cells.